Within a presentation (like beamer) there is very little need to have enumerated captions for images. Regardless, there is no need to always use a (floating) figure environment for including graphics, nor is it necessary to use \caption inside figure.

I would suggest merely using the center environment to center the object and add a piece of text below it to act like the caption:

\documentclass{beamer}
\let\Tiny\tiny% http://tex.stackexchange.com/q/58087/5764
\usepackage{graphicx}
\begin{document}
\begin{frame}
This is text.
\begin{center}
\includegraphics[width=.5\linewidth]{example-image}
This is a rectangle
\end{center}
Here is some more text.
\end{frame}
\end{document}